Formation of a zinc passivation layer on titanium or titanium alloys used in semiconductor processing

ABSTRACT

Embodiments of the current invention describe methods of processing a semiconductor substrate that include applying a zincating solution to the semiconductor substrate to form a zinc passivation layer on the titanium-containing layer, the zincating solution comprising a zinc salt, FeCl 3 , and a pH adjuster.

PRIORITY CLAIM TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 13/335,011 entitled “Formation of a Zinc Passivation layer onTitanium or Titanium Alloys Used in Semiconductor Processing” filed onDec. 22, 2011, which is a Divisional Application of U.S. patentapplication Ser. No. 12/368,110 entitled “Formation of a ZincPassivation layer on Titanium or Titanium Alloys Used in SemiconductorProcessing” filed on Feb. 9, 2009, now issued as U.S. Pat. No. 8,143,164both of which are incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor processing.More specifically, a method of forming a zinc passivation layer ontitanium or titanium alloys used in semiconductor processing as well asthe zincating solution used in this method is described.

BACKGROUND OF THE INVENTION

Titanium and its alloys are highly reactive with oxygen and will rapidlyform an oxidized layer on exposed surfaces when exposed to air ormoisture of any kind. This means that titanium and its alloys willreoxidize very quickly even after the removal of an oxidized layer. Thismakes the formation of coatings or layers of other types of materialsdirectly onto titanium or titanium alloys difficult and layers formed onan oxidized titanium layer may not be uniform because of lack ofadhesion.

The formation of a platinum (Pt) passivating film on titanium nitride(TiN) is of value to the memory applications. Because of the rapidoxidation of TiN after removal of the native oxide a vacuum depositionof Pt onto TiN has been attempted using physical vapor depositiontechniques. The use of a vacuum atmosphere solves the problem ofreoxidation of the TiN in air but PVD is a nonselective deposition,requiring the subsequent removal of platinum from regions other than theTiN and therefore increasing the number of processing steps. Wetprocesses to deposit platinum on TiN are inherently difficult to usebecause TiN bared of its oxide will rapidly oxidize when exposed towater or air in such a non-vacuum environment. The oxide layer may beremoved before the application of the wet processes such aselectrodeposition or electroless deposition. The oxidized layer istypically removed using an acidic formulation that includes hydrofluoricacid (HF). This is detrimental to most substrates because HF is a strongetchant and will attack materials other than the oxide layer. Inparticular, in memory applications, HF will attack materials such assilicon, silicon dioxide, and tungsten. Therefore HF chemistries makethe use of subsequent wet processes difficult. Also, similar to PVD,electrochemical deposition of Pt is nonselective to TiN.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings:

FIG. 1 is a flowchart describing a zincating process according tovarious embodiments;

FIGS. 2A-2F illustrate cross-sections of a substrate at different pointsin the zincating process according to various embodiments.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

Embodiments of the current invention describe methods of forming a zincpassivating layer using a wet process on the surface of a titanium or atitanium-containing layer of a semiconductor substrate. The zincpassivating layer may allow for subsequent aqueous processing to deposita film on the titanium or titanium alloy. In one embodiment the aqueousprocessing may be an electroless deposition process. In one particularembodiment, a substrate having a titanium nitride surface is processedto selectively form a zinc passivating layer on the titanium nitrideportions of the substrate and to subsequently form a platinum layerselectively on the titanium nitride portions using an electrolessplating process.

At block 101 of the flowchart illustrated in FIG. 1, a semiconductorsubstrate is provided that has a titanium or a titanium-containinglayer. The titanium-containing layer may be a titanium alloy such astitanium nitride or titanium aluminum nitride. The titanium alloy may bea combination of titanium and one or more of the following elements:nickel, aluminum, ruthenium, silicon, vanadium, palladium, molybdenum,zirconium, chromium, or niobium. In one particular application, forexample, titanium nitride is used for memory applications in thesemiconductor industry where titanium nitride is valued for its workfunction. In this application, as illustrated in FIG. 2B, the substrateprovided may be a semiconductor substrate 200 such as silicon. Thesemiconductor substrate 200 may also include materials such as silicondioxide as an insulator 210 and tungsten as a bottom electrode material220 over which the titanium-containing material 230 may be formed. FIG.2B is a cross-sectional view of FIG. 2A, cutting through FIG. 2A from Xto Y. FIG. 2A illustrates, for purposes of example to describe anembodiment of the current invention, a top-down view of a semiconductorsubstrate used as a test chip having multiple tungsten lines 220 andover which regions of the titanium nitride 230 have been formed. FIG. 2Billustrates the titanium nitride regions 230 after an oxide layer 235has formed.

At block 102 an oxide layer, such as oxide layer 235 illustrated in FIG.2B, is removed from the titanium-containing layer 230 to leave an oxidefree surface as illustrated in FIG. 2C. Titanium and titanium alloyseasily form an oxide layer on any exposed surfaces when exposed tooxygen. As such, any exposure to the air or moisture will form the oxidelayer. The oxide layer may interfere with the adhesion and uniformity ofmaterials deposited on the titanium-containing layer. Therefore it isbeneficial to remove the oxide layer before the deposition of anymaterial on the titanium-containing layer. The oxide layer may beremoved with either an acidic or an alkaline chemical solution. Theacidic solution may include a mixture of acids and in an embodiment usesoxalic acid (H₂C₂O₄) as the main acid mixed with other acids such assulfuric acid (H₂SO₄), hydrochloric acid (HCl), and phosphoric acid(H₃PO₄). The acid etching solution does not include hydrofluoric acid(HF) because HF is too harsh for application to the types of materialsused in semiconductor processing and memory applications, such assilicon, silicon dioxide and tungsten. In one particular embodiment theacid solution used to etch the oxide layer from the titanium-containinglayer is a mixture of oxalic acid, hydrochloric acid, and phosphoricacid. The oxalic acid is the main etching acid, the hydrochloric acidserves as an etching accelerator, and the phosphoric acid is used toenhance uniform TiN surface etch serves where the concentration range ofoxalic acid is in the approximate range of 10 g/L and 100 g/L of oxalicacid-dihydrate, hydrochloric acid in the approximate range of 100 ml/Lto 500 ml/L of 37 weight % HCl in water, and phosphoric acid in theapproximate range of 50 ml/L and 300 ml/L of 85 weight % phosphoric acidin water. This acid etching solution may have a temperature in the rangeof 40° C. to 95° C. during the etching process and may be applied to thesubstrate for a time period in the range of 30 seconds to 60 minutes. Inone particular embodiment this etching solution has a temperature ofapproximately 80° C. and is applied to the substrate for a time in theapproximate range of 1 minute to 3 minutes. The etching solution may bein the form of a bath and the substrate immersed in the bath.Alternatively the solution may be dispensed or sprayed onto the surfaceof the substrate.

In an alternate embodiment the etching solution may be an alkalinesolution including tetramethylammonium hydroxide (TMAH), a chelatingagent, and an oxidizer. This alkaline solution dissolves the surfaceoxide layer into soluble HTiO₃ ⁻ ions in the aqueous solution. The pH ofthe solution is determined by the TMAH addition and may be in the rangeof pH 12-14, and the alkaline solution may be within the temperaturerange of 20° C.-90° C. The addition of chelating agents such asethylenediaminetetracetic acid (EDTA), ethylenediamine (ED), citricacid, tartaric acid may help induce uniform surface etching. Theconcentration of the chelating agent can vary within the approximaterange of 0.5 millimoles per liter to 10 millimoles per liter. Theaddition of oxidizers such as hydrogen peroxide (H₂O₂), irontri-chloride (FeCl₃), ammonium persulfate ((NH₄)₂S₂O₈), or perchloricacid (HClO₄) may ensure the formulation of soluble HTiO₃-ions. Theoxidizer concentration can be within the approximate range of 10millimoles per liter to 1 mole per liter.

After removing the oxide layer, the surface of the titanium or titaniumalloy is rinsed with an alcohol rinse at block 103 of the flowchart inFIG. 1. The alcohol rinse removes any of the etching solution from thesurface of the substrate and also aids in maintaining the oxide-freesurface. Alcohols that may be used as the rinse include methanol,ethanol, and isopropyl alcohol (IPA) or any mixture of these alcohols.In one particular embodiment, the rinse may be isopropyl alcohol (IPA).The IPA may be applied to the substrate at approximately roomtemperature and for a time in the approximate range of 2 minutes or lessand more particularly for less than 30 seconds. The IPA rinse should beapplied to the substrate immediately after removal of the oxide toprevent any re-oxidation of the titanium or titanium alloy. The IPArinse may be applied to the substrate within approximately 15 secondsafter the application of the oxide etching solution to the substrate. Inan embodiment, the IPA rinse may be applied to the substrate while theoxide etching solution is still on the surface of the substrate so thatthe non-oxidized titanium or titanium alloy is not exposed to any air ormoisture that could cause re-oxidation.

The zincating formulation is then applied to the semiconductor substratehaving a titanium-containing layer to form a zinc passivation layer onthe titanium-containing layer at block 104 of the flowchart of FIG. 1.FIG. 2D illustrates the formation of a zinc passivation layer 240 on thesurface of the titanium containing layer 230. The zincating formulationis formed of a zinc salt, iron chloride (FeCl₃), and an acid. The zincsalt may be zinc sulfate (ZnSO₄) which may be in the form of ZnSO₄.5H₂O.Other zinc salts may also be used such as zinc chloride, (ZnCl₂) andzinc nitrate (Zn(NO₃)₂). The zinc salt provides the zinc needed to formthe zinc layer on the titanium-containing layer of the substrate in thezincating process. The amount of the zinc salt in the formulation has aconcentration in the approximate range of 4 g/L to 200 g/L and moreparticularly in the approximate range of 10 g/L to 40 g/L. The ironchloride (FeCl₃) serves to activate the titanium-containing surface forthe deposition of the zinc layer during the zincating process. Theamount of iron chloride in the zincating formulation may be in theapproximate range of 6 mM to 18 mM. The acid is added to the zincatingformulation to adjust the pH of the solution to be in the approximaterange of 1.9 and 2.4, and more particularly in the approximate range of2.2 and 2.3. Acids that may be used include hydrochloric acid (HCl),H₂SO₄, aminosulfonic acid, citric acid, tartaric acid, sulfamic acid,and oxalic acid, or any mixture of these acids. In one particularembodiment, the acid is a mixture of sulfamic acid and oxalic acid. Theamount of acid added depends on the amount needed to adjust the pH to bewithin the ideal range. This may be an amount of acid in the approximaterange of 10 mM to 50 mM. The addition of the acid also prevents thezincating solution from hydrolysis. If the pH is too high, the zinc mayturn into zinc hydride and precipitate out of the solution. The pH rangeof 1.9 to 2.4 maintains the zinc ions in solution so that zincating mayoccur. The zincating formulation is formulated such that the acid usedas the pH adjuster does not need to be a strong acid, such ashydrofluoric acid, that may etch the substrate.

The zincating formulation may also include a surfactant to increasewettability of the solution on the surface of the substrate. Thesurfactant may be anionic, cationic, or non-ionic. In one particularembodiment, the surfactant may be a cationic compound such ascetyltrimethylammonium bromide (CTAB). The amount of surfactant added tothe solution may be in the approximate range of 10 ppm to 200 ppm. Thezincating formulation may be applied to the substrate by immersion ofthe substrate in a bath of the zincating solution. Alternate methods ofapplication include spraying or dispensing the zincating solution ontothe substrate. In an embodiment, the zincating formulation may beapplied to the substrate while also applying ultrasonic or megasonicenergy to the substrate. One way to do this is by immersing thesubstrate in a bath of the zincating formulation and applying the sonicenergy to the bath. In this embodiment, the ultrasonic energy may have afrequency in the approximate range of 20 kHz to 40 kHz. The ultrasonicenhanced zincating may improve the uniformity of the zinc layer formedon the titanium-containing layer. The use of ultrasonic energy incombination with the application of the zincating formulation may beperformed for a time in the approximate range of 20 seconds to 5minutes, and more particularly for approximately 1 minute. Theapplication of the zincating formulation to the substrate will form avery thin zinc passivation layer that may have a thickness in theapproximate range of 0.3 nm and 10.0 nm and more particularly in theapproximate range of 2.0 nm to 5.0 nm.

In an embodiment, a double zincating process may be performed. Blocks102, 103 and 104 of the process may be repeated to enhance the zincatingprocess. Block 102 is the oxide etching of the substrate surface toremove the oxide from the titanium-containing layer. The second etchingmay remove any titanium oxide that formed after the initial oxideremoval or any oxide that remained after the initial oxide removal. TheIPA rinse of block 103 may then be applied to the substrate to preventthe growth of any new oxide on the titanium-containing layer. Thezincating formulation of block 104 may then be applied again to thesubstrate to form the zinc passivation layer over thetitanium-containing layer. It is theorized that by repeating blocks 102,103, and 104 of the zincating process that it may be possible to form amore uniform and oxide-free zinc passivation layer on the surface of thetitanium-containing layer. The double zincating process may alsoincrease the density of the zinc layer and thus improve the coverage ofthe zinc layer on the titanium-containing layer. Greater control of thethickness of the zincating layer may also be possible using thisprocess, which may also allow for the avoidance of overplating of zinc.The second zincating process may apply the oxide etching solution andthe zincating formulation to the substrate for a shorter time periodthan in the first application. The repetition of the zincating processis not limited to a double zincating process but may be repeated anynumber of times to achieve a multiple zincating process.

At block 105 of the flowchart of FIG. 1, the substrate is rinsed with awater rinse to remove any excess zincating solution after the formationof the zinc passivation layer. The water rinse may be deionized water atroom temperature and may be applied for the time needed to remove thezincating solution from the substrate.

The substrate may be annealed at block 106 of FIG. 1. The thermaltreatment in the form of annealing may be performed at a temperature inthe approximate range of 250° C. and 300° C. for 30 minutes to 1 hour.The annealing may improve the uniformity and density of the zincpassivation layer formed on the titanium-containing layer.

At block 107 of the flowchart of FIG. 1, an aqueous process is appliedto the substrate. Examples of aqueous processes that may be used includeelectroless deposition, electroplating, or a displacement process. Thezincating process forms a zinc passivation layer on atitanium-containing layer that protects the titanium-containing layerfrom oxidation before and during the aqueous process. The aqueousprocess may be used to deposit a metal over the titanium-containingmaterial. In the instance where the aqueous process is an electrolessprocess the zinc layer may act as an activation layer and may thereforebe consumed in the process resulting in the formation of the newlydeposited layer directly on the titanium-containing film. An example ofthis is illustrated in FIG. 2E where a layer of platinum iselectrolessly deposited on the titanium nitride layer 230, where thezinc layer 240 has served as an activation layer for the electrolessdeposition and is consumed in the process. FIG. 2F illustrates the topdown view of the substrate of FIG. 2E.

In one embodiment, platinum may be selectively plated over a titaniumnitride film in semiconductor processing as part of the building of amemory device. In this embodiment the zinc passivation layer is formedon a titanium nitride layer by electroless deposition where the zincpassivation layer also acts as an activation layer. The zinc layer istherefore consumed by the electroless process to form a layer ofplatinum directly on the titanium nitride. In one particular embodiment,the electroless plating solution to plate the platinum film overtitanium nitride may be formed of the following components in deionizedwater. The platinum supply chemical is pre-treated chloroplatinic acidwhere the concentration of platinum in the plating bath is in theapproximate range of 2 mM and 30 mM. The reducing agent is hydrazine ina concentration in the approximate range of 0.1M and 1M. This hydrazineconcentration in combination with the complexing agent hydroxylamine andthe stabilizer additives enables continuous, self-initiated, platinumdeposition operation at bath temperatures of up to 80° C. without theneed of intermittent hydrazine addition. The accelerator is sulfamicacid, also known as aminosulfonic acid, in a concentration in theapproximate range of 0 to 0.5M. The primary stabilizer is5-sulfosalicylic acid in a concentration in the approximate range of 2mM to 20 mM, and the secondary stabilizer is EDTA having a concentrationin the approximate range of 1 mM to 10 mM. In this embodiment, theelectroless plating solution is applied at a temperature of 20-80° C.and at a pH in the approximate range of 8 and 12. The electrolessplating solution is applied to the substrate by submerging the substratein a bath of the solution for a time of approximately 0.5 and 2.0minutes to form a platinum layer having a thickness of 100 Å to 400 Å.

The electroless plating solution may be applied to the surface of thesubstrate 200 in any conventional manner to form the platinum film 250.Methods of applying the electroless plating solution include, but arenot limited to, substrate immersion into a plating bath tank, and bathsolution dispensation onto the substrate from a dispensing nozzle orshower head connected with a bath reservoir and flow controller. In oneembodiment the electroless plating solution is a bath and the substrateis submerged for a particular amount of time. The electroless platingbath solution may have a temperature in the range of 20° C. to 85° C.and a pH in the range of 8 to 12. The plating solution may be applied tothe substrate for an amount of time in the range of approximately 30seconds to 5 minutes, although this can vary depending on how thick of aplatinum film is required. The amount of time that the substrate isexposed to the plating solution depends on the thickness desired for theplatinum film being formed on the copper film. The choice of thicknessof the platinum film depends on the particular application. The platinumfilm may be a passivation layer over titanium or titanium-containinglayers in various semiconductor based devices in both memory and logicapplications. For an electrode application for memory chips thethickness of the platinum film may be 50-200 Å. A selectively formedplatinum film is valuable as a passivation layer atop a bottom electrodefor certain memory devices. For example, the memory device may be ametal-insulator-metal (MIM) structure.

The process outlined in the flowchart of FIG. 1 may be followed throughin any number of different ways. In one exemplary embodiment, the methodmay be a single zincation process having the following recipe:

-   -   a) Oxide etching to remove the oxide layer from a titanium        nitride layer using an acidic etching solution having a        temperature of approximately 80 C and is applied to the        substrate for approximately 5-7 minutes;    -   b) Applying an IPA rinse at room temperature for approximately        15 seconds;    -   c) Applying a zincating solution at room temperature for a time        in the approximate range of 20 seconds to 5 minutes to form a        zinc passivation layer selectively on the titanium nitride        layer;    -   d) Rinsing the substrate with a deionized water rinse at room        temperature for approximately 30 seconds;    -   e) Depositing a platinum layer selectively on the        titanium-containing layer using electroless deposition;    -   f) Rinsing the substrate with deionized water and drying the        substrate.

In another embodiment, the method may use a double zincating process asfollows:

-   -   a) Oxide etching to remove the oxide layer from the        titanium-containing layer using an acidic etching solution at 80        C for approximately 2 minutes;    -   b) Applying an IPA rinse to the substrate at room temperature        for approximately 15 seconds;    -   c) Applying a zincating formulation to the substrate at room        temperature for a time in the approximate range of 20 seconds to        1 minute;    -   d) Applying an acidic etching solution at 80 C to the substrate        for approximately 2 minutes;    -   e) Rinsing the substrate with the IPA rinse at room temperature        for 15 seconds;    -   f) Applying the zincating formulation to the substrate at room        temperature for approximately 20 seconds;    -   g) Applying a deionized water rinse to the substrate at room        temperature for 30 seconds;    -   h) Depositing a platinum layer on the titanium-containing layer        using electroless deposition; and    -   i) Rinsing the substrate with deionized water and drying the        substrate.

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

What is claimed is:
 1. A method of processing a semiconductor substrate,comprising: providing the semiconductor substrate comprising atitanium-containing layer; formulating a zincating solution comprising azinc salt, FeCl3, and a pH adjuster; using the pH adjuster to adjust thepH of the zincating solution to within a range of 1.9 and 2.4; applyingthe zincating solution to the semiconductor substrate to form a zincpassivation layer on the titanium-containing layer; wherein the pHadjuster comprises an acid.
 2. The method of claim 1, further comprisingrinsing the substrate after applying the zincating solution.
 3. Themethod of claim 1, wherein applying the zincating solution comprisesapplying a first zincating formulation to form a first zinc layer,rinsing the first zinc layer with isopropyl alcohol, and applying asecond zincating formulation to form a second zinc layer.
 4. The methodof claim 1, further comprising applying an aqueous process to thesubstrate after forming the zinc passivation layer.
 5. The method ofclaim 4, wherein the aqueous process is an electroless plating process.6. The method of claim 1, further comprising etching an oxide layer froma surface of the substrate before applying the zincating solution to thesubstrate.
 7. The method of claim 6, wherein the oxide layer is etchedby an alkaline solution comprising tetramethylammonium hydroxide (TMAH),ethylenediaminetetraacetic acid (EDTA), and hydrogen peroxide (H₂O₂). 8.The method of claim 1, further comprising annealing the zinc passivationlayer after it is formed.
 9. A method of processing a semiconductorsubstrate, comprising: providing the semiconductor substrate comprisinga titanium-containing layer; formulating a zincating solution comprisinga zinc salt, FeCl3, and a pH adjuster, wherein the pH adjuster comprisesan acid; using the pH adjuster to adjust the pH of the zincatingsolution to within a range of 1.9 and 2.4; applying the zincatingsolution to the semiconductor substrate to form a zinc passivation layeron the titanium-containing layer; and applying an electroless processfor depositing a metal layer to the semiconductor substrate comprisingthe zinc passivation layer, wherein the zinc passivation layer isconsumed and resulting in a formation of the metal layer depositeddirectly on the titanium-containing layer.
 10. The method of claim 9,further comprising rinsing the substrate before or after applying thezincating solution.
 11. The method of claim 9, wherein applying thezincating solution comprises applying a first zincating formulation toform a first zinc layer, rinsing the first zinc layer with isopropylalcohol, and applying a second zincating formulation to form a secondzinc layer.
 12. The method of claim 9, further comprising etching anoxide layer from a surface of the substrate before applying thezincating solution to the substrate.
 13. The method of claim 9, furthercomprising annealing the zinc passivation layer after it is formed. 14.A method of processing a semiconductor substrate, comprising: providingthe semiconductor substrate comprising a titanium-containing layer;applying an oxide etching solution to a surface of thetitanium-containing layer; rinsing the semiconductor substrate with analcohol while the oxide etching solution is still on the surface;applying a zincating solution to the semiconductor substrate to form azinc passivation layer on the titanium-containing layer, the zincatingsolution comprising a zinc salt, FeCl₃, and a pH adjuster, wherein thepH adjuster comprises an acid; rinsing the substrate after the formationof the zinc passivation layer; annealing the substrate; and applying anelectroless process for forming a metal layer on the semiconductorsubstrate.
 15. The method of claim 14, wherein applying the zincatingsolution comprises applying a first zincating formulation to form afirst zinc layer, rinsing the first zinc layer with isopropyl alcohol,and applying a second zincating formulation to form a second zinc layer.16. The method of claim 14, wherein the titanium-containing layercomprises a titanium nitride layer.
 17. The method of claim 14, whereinthe metal layer comprises a platinum layer.
 18. The method of claim 14,further comprising adjusting the pH of the zincating formulation usingthe pH adjuster to be within a range of 1.9 and 2.4 before applying thezincating formulation to the semiconductor substrate using the pHadjuster.